1. Field of the Invention
The present invention relates to a color cathode ray tube (CRT), and more particularly to a tension mask frame assembly for a color CRT having an improved frame for supporting a mask to which a tensile force is applied.
2. Description of the Related Art
In a typical color CRT, three electron beams emitted from an electron gun pass through electron beam passing holes of a shadow mask having a color selection function and land on phosphors of red, green, and blue of a fluorescent film formed on a screen surface of a panel so that the fluorescent substances are excited to form an image.
In a color CRT forming an image, the mask having a color selection function is classified as a dot mask used in a monitor of a computer or a slot mask (or a slit mask) used in a television. The mask is fixed to the frame and installed to the panel of the CRT.
The slot mask is further classified as a forming mask designed to have a curvature corresponding to that of the screen surface considering the screen surface and the landing of the deflected electron beams, or a tension mask supported so that a tensile force can be applied considering a flat screen surface of a panel to correct distortion of an image and enlarge a view angle of the screen.
A coupling structure of the mask and the frame in which a tensile force is applied to the mask is disclosed in Japanese Patent Publication Nos. sho 59-18825 (1984) and sho 59-16626 (1984).
A mask frame assembly to prevent the amount of expansion of the frame in a direction in which a tensile force is applied to the mask is disclosed in U.S. Pat. No. 5,111,107.
The disclosed mask frame assembly is shown in FIG. 1. As shown in the drawing, the mask frame assembly includes a pair of support bars 11 installed to face each other, a pair of elastic support members 12 installed between the support bars 11 to support the support bars 11, a mask 13 supported by the support bars 11, and metal members 14 attached to surfaces of the respective elastic support members 12 opposite to the side where the mask 13 is installed and having a thermal expansion coefficient greater than that of the elastic support members 12.
In the mask frame assembly 10, even though metal members 14 are attached, deterioration of tensile forces occurs in the mask 13 after heat treatment so that the effect of the metal members 14 varies according to the distribution of tension in the x- and y-directions.
A color CRT having a mask frame assembly to reduce the decrease of tension of a mask during a blackening process and a heat treatment is disclosed in Japanese Patent Publication No. hei 11-317176.
The color CRT includes a pair of support bodies facing each other and a color selection electrode where a grid is suspended on a frame made of a pair of elastic support members installed between the support bodies. A control member having a lower thermal expansion coefficient in a low temperature area and a higher thermal expansion coefficient in a high temperature area, compared to the thermal expansion coefficient of the elastic support body, is fixed to a surface opposite to the grid of the elastic support member, or a control member having features opposite to the above control member is fixed to the elastic support member at the side corresponding to the grid.
The color selection apparatus of a color CRT having the above structure cannot solve the above problem of properly supporting a mask because the control member is attached to the elastic support member having a different thermal expansion coefficient.
Another conventional tension mask frame assembly attempting to solve the above problem of properly supporting a mask discloses a pair of first and second support members separated by a predetermined distance, first and second elastic members installed between the first and second support members to support the first and second support members and having support portions fixed to the first and second support members and connection portions connecting the support portions. The mask is installed such that tension is applied to the first and second support members and has a plurality of electron beam passing holes. A compensation member is installed to connect the first and second support members or support portions between the connection portions and the mask and is made of a material having a thermal expansion coefficient lower than that of the first and second elastic members.
In the above tension mask frame assembly, by installing the compensation member so as to connect both end portions of the first and second elastic members or end portions of the first and second support members between the upper portion of the connection portions of the first and second elastic members and the lower portion of the mask, the thermal expansion of the first and second elastic members acts as a tensile force to the mask so that the mask is prevented from being plastically deformed.
However, in the tension mask frame assembly having the above structure, the tension of the tension mask changes due to differences in the thermal expansion amounts of the frame, the support portion, and the tension mask occurring after heat treatment. The tension of this conventional deformed tension mask cannot be adjusted after the heat treatment.